Which phase of clinical trials involves the largest number of subjects?

The correct choice is based on the fact that Phase III clinical trials are designed to confirm the efficacy and safety of a treatment in a larger patient population. This phase typically involves several hundred to several thousand subjects, significantly more than in the earlier phases of clinical trials.

Phase I trials focus primarily on safety and typically involve a small group of healthy volunteers—often fewer than 100 participants. The main goal here is to assess how the drug is metabolized and its safety profile with minimal focus on efficacy.

Phase II trials expand the research to a larger group, often involving several hundred patients, to further evaluate the drug's effectiveness and monitor side effects. However, this is still less than what is generally seen in Phase III trials.

Phase IV, or post-marketing surveillance, occurs after a drug has been approved for public use. While this phase involves large populations over time, it is not a part of the clinical trial process aimed at securing regulatory approval; rather, it focuses on ongoing safety monitoring and further evaluation of long-term effects in the real world.

Thus, Phase III trials encompass the largest number of subjects involved in the clinical trial process, aiming to ensure the drug's benefits outweigh its risks across a broader, more diverse population.